Powerful comet activity pushes back solar wind

The event captured in three chronological images – the image on the left was captured at 13:06 GMT, middle 13:24 GMT and right 13:42 GMT(Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Rosetta has detected a
powerful jet of activity emitted from the comet
67P/Churyumov–Gerasimenko (67P). The force of the outburst, which
is believed to be travelling at 10 m per sec (32 ft per sec), was
strong enough to temporarily repel the solar wind – a constant stream
of charged particles emanating from the Sun, that work to convey our
star's magnetic field across the solar system.

Both comet and
spacecraft made their closest approach with the
Sun, otherwise known as perihelion at 02:03 AM GMT. The Rosetta science team expected
extreme comet activity in the weeks following perihelion, and note
that outgassing activity would be unpredictable even before, but the
sheer power of the recent activity display appears to have caught the
team by surprise.

"This is the brightest jet we’ve seen so far,"
states OSIRIS team member Carsten Güttler, of the Max Planck
Institute for Solar System Research, Göttingen, Germany. "Usually,
the jets are quite faint compared to the nucleus and we need to
stretch the contrast of the images to make them visible – but this
one is brighter than the nucleus."

Rosetta, which is in the process of transitioning to a safer
orbit of 300 km (186 miles), was able to detect the outburst
from a distance of 186 km (116 miles) with
a number of its onboard instruments. While being showered in the
ejected dust particles, ROSINA, the
element of the probe's scientific suit designed to map the comet's
atmosphere and ionosphere, recorded significant structural and
compositional deviations in 67P's coma.

This image captured April 12 shows the origin of the jet on the comet's nucleus in a small red circle(Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

The orbiter's OSIRIS camera captured a sequence of images of 67P
during the episode, first capturing the jet at 13:24GMT, with the
outgassing subsequently appearing to subside a mere 18 minutes later.
However, whilst the jet itself may have been short-lived, its effects
lingered for some time after, with Rosetta's GIADA instrument
recording 30 dust hits on the spacecraft per day, a significant
increase from the standard 1-3 per day usually recorded.

As the event unfolded, the gas and dust being expelled from the comet
pushed back the solar wind. Due to the fact that the comet itself is
not magnetized, Rosetta was able to make detailed studies of how this
solar wind interacted with 67P using its onboard Plasma Consortium
Magnetometer, as any magnetic field readings detected could only be
attributed to the solar wind.

"The solar wind
magnetic field starts to pile up, like a traffic jam, and eventually
stops moving towards the comet nucleus, creating a magnetic
field-free region on the Sun-facing side of the comet called a
'diamagnetic cavity'," explains Charlotte Götz, science team
member for Rosetta's magnetometer instrument at the Institute for
Geophysics and extraterrestrial Physics, Braunschweig, Germany.

The effect only lasted for a few minutes, but managed to push the
solar wind back as far as 186 km (116 miles) from 67P's nucleus, with
the event providing a valuable insight into the power of the comet's
activity as it nears perihelion.